Methods of sputter deposition of materials

ABSTRACT

Spirally-grooved and plain thrust-plates of an aerodynamic gaslubricated bearing are both manufactured by sputter-deposition of a layer of wear-resistant target-material on to flat-disc substrates. Initial sputtering removes material from the substrates and replaces it partially with an alloy of target- and substrate-materials, the sputtering from the substrates being reduced progressively to zero as sputtering from the target is increased progressively to a full value. The content of targetmaterial increases progressively through the alloy-layer up to the final upper layer of target-material alone.

Sates 21 iatent n91 eardmore et a1.

METHODS OF SPUTTER DEPOSITION OF MATERIALS [75] Inventors: Geoffrey Beardmore, C heltenham;

Hugh N. Evans, Bristol, both of England,

,[73] Assignee: Smiths Industries Limited, London,

England [22] Filed: July 8, 1971 [21] Appl. No.: 160,913

Related U.S. Application Data Y [63] Continuation of Ser. No. 786,892, Dec. 26, 1968,

, abandoned.

' Foreign Application Priority Data Dec. 29, 1967 Great Britain 59139/67 [52] U.S. Cl. 204/192 [51] Int. Cl. C23c 15/00 [58] Field of Search 204/192 [56] References Cited UNITED STATES PATENTS 3,021,271 2/1962 .Wehner 204/192 3,324,019 6/1967 Laegreid et a1 204/192 11] 3,869,368 [451 Mar. 4, 1975 3,479,269 11/1969 Byrnes etal. ..-204/192 3,576,729 4/1971 Sigournay et: a1. ..204/29 8 FOREIGN PATENTS OR APPLICATIONS 520,592 4/1940 Great Britain 1,072,742 6/1967 Great Britain 1,100,447 1/1968 Great Britain Primary Examiner-Howard S. Williams Assistant Examiner-D. R. Valentine Attorney, Agent, or Firm-Poll0ck, Philpitt & Van

Sande- [57] ABSTRACT spirally-grooved and plain thrust-plates of an aerodynamic gas-lubricated bearing are bothmanufactured by sputter-deposition of a layer of wear-resistant tar- I duced progressively to .zero as sputtering from the target is increased progressively to a full value. The content of target-material increases progressively through the alloy-layer up to the final upper layer of targetmaterial alone.

2 Claims, 8 Drawing Figures bombardment of a target, the bombarding ions being directed to the target (normally by means of an electric field) from a plasma. It is common practice to arrange for creation of the plasma by collision of an electron beam with the atoms or molecules of a gaseous atmosphere, the substrate upon which deposition is to take place being then conventionally (but not necessarily) positioned directly opposite the target across the electron-beam path.

The mechanical integrity of the deposit on the substrate is very dependent upon the degree of adhesion between the deposited material and the material of the substrate. This factor is especially important in the case of thick deposits that are confined to small areas of contact with the substrate, and becomes critical where such deposits are subject to shearing forces.

It is an object of the present invention to provide a method of sputter-deposition that may be used to provide deposits of improved mechanical integrity.

According to one aspect of the present invention there is provided a method of sputter-deposition of material from a target on to a substrate, wherein material is sputtered from the substrate such that at least part of material subsequently deposited on the substrate in the method, is a combination of targetand substratematerials.

The sputtering of material from the substrate may be carried out concurrently with sputtering of material from the target. In these circumstances the rate of sputtering of material from the target on to the substrate may be increased progressively with time as the rate of sputtering from the substrate is decreased, so as to provide a deposit-in which there is a graded increase in concentration of the target materia'l as the deposit builds up on the substrate. Such gradation from the material of the substrate acts to enhance adhesion between the deposit and the substrate.

The method of the present invention is readily applicable to the deposition of hard, wear-resistant material on a substrate that is formed of a less hard material. In these circumstances the substrate may be readily preshaped by conventional machining methods to have a surface'machined to a close tolerance (for example, of flatness). Deposition of the hard material by the method of the present invention, thereafter allows a hardsurface to be readily formed on the machined surface to substantially the same tolerance as the machined surface and with the advantageous characteristic of good. adhesion to the substrate.

This invention, especially in the context 'of the immediately-preceding paragraph, is applicable to the manufacture of bearing-members for gas-lubricated bearings. Although the invention is not limited to this specific application and may be used in the manufacture of other articles, there is provided in accordance with another aspect of the invention a bearing-member that has a bearing-surface formed of a first, wearresistant material and comprises a substrate-member of a second material, a layer of an alloy composed of said first and second materials sputter-deposited on said substrate-member, and an upper layer of said first material deposited on the alloy-layer to provide said bearing-surface, the concentration of said second material in said alloy-layer decreasing through its thickness from the substrate-member to the upper layer.

A method of sputter-deposition, together with articles manufactured using such method, according to the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is-a part-sectional side-elevation of a form of aerodynamic gas-lubricated bearing that comprises two thrust-plates each manufactured using the sputterdeposition method of the present: invention;

FIG. 2 is a plan view of one of the two thrust-plates of the bearing shown in FIG. 1, the section of FIG. I being taken on the line I-I of FIG. 2;

FIGS. 3 and 4 are respectively a sectional representa- .tion and an external view of apparatus used in the sputter-deposition method of the presentinvention; and

FIGS. 5 to 8 are diagramatic representations illustrative of successive stages in the sputter-deposition method performed using the apparatus of FIGS. 3 and 4.

The form of aerodynamic gas-lubricated bearing shown in FIG. 1 is for use in the provision of a rotational mounting at either end of the rotor of a gyroscope, and the method to be described is used in the batch-production of both thrust-plates of the bearing.

Referring to FIGS. 1 and 2, the aerodynamic bearing comprises two circular plates 1 and 2 that have opposed bearing-surfaces 3 and 4 respectively. The surface 3 of the plate 1 is provided with a series of shallow grooves 5 of logarithmic-spiral pattern, extending from the edge of the plate 1 to a central land 6, whereas the surface 4 of the plate 2 is plain. Relative rotation between the plates 1 and 2 in the sense to cause air to be dragged inwardly along the grooves 5 establishes pressure that balances the axial load exerted between the plates l, and 2. Since this balance is established with a spacing of only a few tenths of a thousandth of an inch between the plates-1 and 2, it is essential for the surfaces 3 and 4 to be optically flat to within onelight band (that is to say, to within 10* micro-inches).- The surfaces 3 and 4 additionally'need to be resistant to wear and other damage arising from their contact with one another when the gyroscope-rotor is stationary and from the rubbing of one upon the other when rotation begins.

The requirement for resistance to wear and other damage implies hardness and this, where conventional machining methods are contemplated, tends to prejudice the provision of accurate surface-flatness and also the provision of the accurate grooving required in the case of plate all the requirements are readily met in the present instance, however, using sputter-deposition of a layer of a hard, wear-resistant material on the optically-flat face of a substrate that is formed of a material that more-readily lends itself to machining. In this specifrc example the substrate is a circular disc of a nickelbase alloy known as MONEL-K-SOO, such material, which has a high nickel content and contains a significant proportion of copper, being readily machinable to provide on both faces a degree of flatness at least as 3. high as that required of the relevant surface 3 or 4. The wear-resistant material used is tungsten carbide and a layer of accurately-controllable thickness is deposited on one of the flat faces of the substrate by the sputterdeposition method to be described. In the case of plate 1, the face of the substrate is masked where the grooves 5 are required, during the sputtering method, the masked areas thereby forming optically-flat bases to the grooves-5 within thedeposited layer.- No masking of the substrate is required in the case of the plain plate 2, but otherwise the method of sputter-deposition used is the same as that for plate 1.

The method of sputter-deposition of the tungsten-- carbide layer is performed using the apparatus of FIGS. 3 and 4, a batch of four substrates being dealt with at a time.

Referring to FIGS. 3 and 4, the apparatus has an anode 11 and a filamentary-tungsten cathode 12 that are spaced from one another along the longitudinal axis 13 of an air-tight enclosure 14. A metal plate 15, which has a central aperture 16 and is positioned within the enclosure'14 adjacent the cathode 12, carries a target 17 that is composed of tungsten-carbide particles compacted in a matrix of cobalt. A stainless-steel jig 18 that is recessed to receive the individual substrates of the batch is positioned opposite the target 17 across the axis 13.

The windings of two toroidal electromagnets 19 and 20 encircle the enclosure 14 coaxially,.adjacent the plate and anode 11 respectively, and the whole is embraced by an aluminium framework 21. The framework 21 comprises two aluminium end-pieces 22 and 23 that are of hexagonal form and encircle the enclosure 14 adjacent the magnets 19 and respectively, and six aluminium rods 24 that interconnect the endpieces 22 and 23 and carry respective toroidal windings 25. The windings 25 are inductively coupled to the closed electrical paths provided by the aluminium framework 21; these paths extend lengthwise of the rods 24 to constitute single-turn secondariescoupled to the windings 25 and adapted to provide a magnetic field acting transversely, and substantially normally, to the electron-beam path along the axis 13.

Before the sputtering apparatus is used, its component-parts, together with the substrates on which deposition is to take place, are all thoroughly cleaned. The cleaning process involves firstly the three successive steps of ultrasonic cleaning in a solution of a liquid detergent (TEEPOL, for example) in de-ionised water,-

vapour cleaning in trichloroethylene, and vapour cleaning in iso-propyl alcohol. The substrates are then loaded in the jig 18 and the apparatus re-assembled with the optically-flat faces of the substrates upon which deposition is to take place exposed to the target 17. Where grooved plates of the form of theplate 1 are to be manufactured the substrates are loaded in the jig 18 with the exposed faces appropriately,maskedwhere the grooves are required. All steps in the loading of the jig 18 and re-assembly of the apparatus are carried but taking care not to handle any of the parts withcontaminated tools or uncovered hands.

The enclosure 14 is next sealed and pumped down to a pressure of 2 X 10' torr by means of a pump (not shown). Argon gas is then admitted through a valve (not shown), spaced in the enclosure 14' from the vent to the pump, so as to sweep through the enclosure 14 (with the pump still running) to increase the pressure m3 X 1O- torr, and ensure a-clean argon atmosphere within the enclosure 14. Under these conditions voltage (for example, of 75 volts) is applied between the anode 11 and cathode 12 sufficient to'strike'an are he tween them to finish the cleaning-up process. No voltage is applied between the cathode l2 and the target 17 or jig 18 at this time, so ionic bombardment does not take place.

At the end of the complete cleaning-up process the supply of argon is reduced so that the pressure within the enclosure 14 thereby falls to between 8 X l0 and 5 X 10 torr. The six windings 25 are then energized with alternating electric current having a frequency of fifty cycles per second and supplied from a three-phase source, so that the single-turn secondaries provided by the aluminium framework 21 produce a radiallydirected magnetic field that rotates about the axis 13. This rotating magnetic field is maintained, and direct current is supplied to each of the electromagnets l9 and 20, throughout the whole sputtering process. During the process the anode 11 is maintained at a positive potential with respect to the cathode 12 so as to set up a substantially cylindrical plasma-column extending lengthwise of the axis 13. The axial magnetic field generated by the electromagnets 19 and 20 provides in this respect a degree of focussing of the electron beam emitted by the cathode 12 (the electromagnet 20 appears also to exert a stabilizing effect on the plasmacolumn); the action of the rotating magnetic field is to improve the uniformity of deposition obtained on the substrates.

Initially in the process there is no voltage applied between the target 17 and cathode 12; voltage is however applied between the jig 18 (and thereby, each of the four substrates) and cathode 12. The sense of application of this latter voltage is such that the substrates are negative with respect to the cathode l2 and accordingly such as to result in ionic bombardment of them from the plasma-column. Bombardment of the exposed areas of the substrates causes sputtering of the nickelalloy material, and some of this material is deposited o the target 17.

From this initial condition there is now established a voltage between the target 17 and cathode 12. The

' voltage is applied in the sense to render the target 17 negative with respect to the cathode 12 and thereby es tablish the condition for ionic bombardment of the target 17 and sputtering of material therefrom. This voltage throughout the first 15 minutes of the complete process is increased in steps up to the value initially applied between the substrates and cathode 12. As this increase'in voltage progresses so the voltage between the substrates and cathode 12 is decreased in steps to reach zero at the end of the 15 minute period. As the target-voltage is increased and the substrate-voltage decreased so the rate of sputtering of material from the target increases progressively from zero and that from the substrates decreases progressively to zero. Thus a differential process of concurrent sputtering takes place, with material from the substrates deposited onthe target 17 and material from the target 17 deposited on the substrates, and with the rate of deposition on the substrates increasing progressively to a maximum throughout the.l5 minute period. Deposition on the substrates is continued at this maximum rate from the end of the 15 minute period until the desired thickness s of deposit has been obtained, but no further deposition on the target 17 takes place.

Illustration'of the effect of the differential sputteringprocess is provided by FIGS. to 8. These figures show diagramatically the conditions that apply at a part of the optically-flat face 30 of a nickel-alloy substrate 31 during successive stages of the complete sputtering process. The illustrations relate to the formation of a thrust-plate having the form of the grooved plate 1, and in this respect a mask 32, of annealed berillium-copper, is shown clamped tightly to the face 30 so that this face is only partially exposed to the target 17. I

FIG. 5 illustrates the condition that exists some three minutes after the beginning of the fifteen-minute period. Material of the substrate 31 is still being removed by strong ionic bombardment of the unmasked area of the surface 30, leaving a crater 33. The ionic bombardment of the target 17 up to this time is too weak to have any significant effect at the substrate 31.

After 5 minutes from the beginning of the 15 minute period the ionic bombardment of the target 7 has been increased to such an extent that, as illustrated by FIG. 6, there isa small degree of deposition of material from the target 17 within-the crater 33. Some of the nickelalloy material earlier deposited on the target 17 from the substrates 30 is returned with tungsten-carbide material from the target 17 in this deposition. Ionic bombardment of the substrate 31 with consequent removal of material therefrom, continues but at a reduced rate,

and the overall effect is to result in the introduction of tungsten-carbide material 34 in low concentration, and alloyed with the nickel-base alloy, at the bottom of the still growing crater 33. a

Growth of the crater 33 ceases some minutes from the beginning of the IS minute period. As illustrated by FIG. 7, the rate of deposition of material from the target 17 following this, exceeds the rate of removal of material from the substrate 31. There is consequently a general build-up of combined targetand substratematerials within the crater 33, with the concentration of the tungsten-carbide material 34 in the resultant alloy increasing progressively with'time as the ionic bombardment of the target 17 increases and that of the substrate 31 decreases to zero.

Eventually, at the end of the minute period, the ionic bombardment of the substrate 31 ceases, and from this point onwards the material deposited on the substrate 31 is solely the tungsten-carbidermaterial 34 of the target 17, as illustrated by FIG. 8. Since the net build-up of material in the crater 33 begins only some 5 minutes before the end of the 15 minute period, the

' level 35 of deposit at the end of the period is still some 5 micro-inches below the face 30, andit is from this I level 35- that the straightforward deposition of the target-material proceeds. Deposition of material from the target 17 continues until the required coating-thickness above the face 30 is obtained.

The target 17 is composed of tungsten-carbide particles compacted in a cobalt matrix, and although only 3 per cent of the total weight is attributable to cobalt, it has been foundthat the material initially deposited on the substrate 31 from the target 17 has a higher cobelt-content than this. However, as deposition continues there is a reduction in cobalt-content to below 3 per cent by weight, and the final coating tends to have a tungsten-carbide content greater than that (97 per cent by weight) of the target 17.

Accurate control of the coating thickness is obtained simply by regulating the time for which sputtering is continued. When the desired thickness has been obtained, the voltages applied to the apparatus are switched off and the whole allowed to cool. The enclo-- sure 14 is opened when all is cool, and the coated substrates (with such masks as relevant, taken off) are then I removed from the jig 18. After polishing they are ready for use. p

The polishing is applied simply to remove minor surface imperfections causedin certain circumstances by expitaxial' dendrite growths, and consists of rotating the coated substrate at high speed in contact with a soft cloth that is impregnated with a very fine diamond compound. The final form of the article is sufficiently accurately related to that of the machined substrate and the period of deposition, as to obviate any need for machining. The configuration of the grooving in the plates manufactured to the form of plate 1, is determined accurately by the masking alone; the grooves 5 have well-defined side-walls, and optically-flat bases provided by the pre-machined face of the substrate.

The fact that the tungsten-carbide coating extends below the surface of the substrate with gradation of concentration of such material with depth, is of advantage in providing a keying effect. Such keying is of especial advantage where,-as with the plate 1, the coating is divided into narrow projections from the surface of the substrate. In addition, the sputtered coating tends to be less porous than coatings formed in other ways, and the initial sputtering from the substrate ensures that the surface upon which deposition of the coating is made is entirely clean and nascent. These effects tend to make the coating more secure mechanically, especially in shear, as compared with coatings laid down directly upon a substrate-surface.

The coated surface of each finished article tends to be slightly convex. This is believed to be due to the ionic bombardment applied to each substrate, such bombardment causing plastic deformation at the exposed surface with resultant warping. The convexing or crowning tends to be of advantage in the case of the thrust-plates 1 and 2, and is sometimes incorporated specifically as a design feature for. reducing ringing (that is to say, adhesion) of the opposed bearingsurfaces. The crowning can be corrected partially, or

completely eliminated, simply by reversing each substrate within the jig 18 after the deposition process and repeating the process on the second face; where the crowning is to be eliminated completely the whole process is repeated, but otherwise only part, the length of time involved being dependent upon the extent of correction required.

The method of deposition has been described above in relation to production of only four articles at a time, but it may readilybe applied to production of very I large batches,-for example, of such articles. In this respect a plurality of targets and jigs may be employed in place of the single target 17 and jig 18 of the apparatus of FIGS. 3 and 4, such targets and jigs being disposed alternately with one another in a circle about the axis 13. Additionally, it may be arranged that deposi-' tion takes place concurrently, or successively, from targets of differing materials, in order to provide coatings formed as desired, of such materials. Furthermore, de-

position may be made on other than flat surfaces; for example, deposition may be made on a cylindrical surface by arranging that the substrate in this case is rotated to present all portions of the surface successively and repeatedly to the target during the process.

The present invention is applicable to the deposition of electrically non-conductive, as well as electrically conductive, materials. Furthermore, attraction of the ions from the plasma may be effected by means of an alternating electric field (for example of radio frequency), this technique being applicable especially to -those circumstances in which an electrically nonconductive material is to be deposited.

in connection with the use of a rotating magnetic field during sputtering, attention is directed to copending U.S. Patrapplication Ser. No. 733,669 filed May 31, 1968, now U.S. Pat. No. 3,576,729, in the' names of N. L. Sigournay and H. N. Evans. This application includes disclosure of sputtering apparatus similar to that shown in F I68. 3 and 4 of the present appli- I as to sputter substrate material from the surface thereof,

B. Thereafter applying a negative voltage to a target formed of said particular material which voltage increases in amplitude while concurrently the voltage applied to the substrate is decreased to thereby increase the rate of sputtering from the target progressively from zero while concurrently the rate of sputtering from the substrate decreases progressively to zero,

C. And thereafter continuing to apply said negative voltage to the target to sputter therefrom onto the substrate while the voltage on the substrate is maintained at substantially zero voltage until the desired amount of target material is deposited on the substrate in accordance with the desired thickness of the projection.

2. The method of claim 1 wherein the sputtering of I the substrate material from the surface of the substrate occurs selectively only over a predetermined portion of such surface to produce a cavity therein, and the sputtering of substrate and target material from the target onto the substrate is effective to fill such cavity and eventually to deposit material thereon to a level above that of the original substrate surface. 

1. IN A METHOD OF SPUTTER DEPOSITION OF A PARTICULAR MATERIAL FROM A TARGET ONTO A SURFACE OF A SUBSTRATE TO FORM THEREON A PROJECTION FROM SAID SURFACE COMPRISING THE STEPS OF: A. APPLYING A NEGATIVE POTENTIAL TO SAID SUBSTRATE SO AS TO SPUTTER SUBSTRATE MATERIAL FROM THE SURFACE THEREOF, B. THEREAFTER APPLYING A NEGATIVE VOLTAGE TO A TARGET FORMED OF SAID PARTICULAR MATERIAL WHICH VOLTAGE INCREASES IN AMPLITUDE WHILE CONCURRENTLY THE VOLTAGE APPLIED TO THE SUBSTRATE IS DECREASED TO THEREBY INCREASE THE RATE OF SPUTTERING FROM THE TARGET PROGRESSIVELY FROM ZERO WHILE CONCURRENTLY THE RATE OF SPUTTERING FROM THE SUBSTRATE DECREASED PROGRESSIVELY TO ZERO, C. AND THEREAFTER CONTINUING TO APPLY SAID NEGATIVE VOLTAGE TO THE TARGET TO SPUTTER THEREFOM ONTO THE SUBSTRATE WHILE THE VOLTAGE ON THE SUBSTRATE IS MAINTAINED AT SUBSTANTIALLY ZERO VOLTAGE UNTIL THE DESIRED AMOUNT OF TARGET MATERIAL IS DEPOSITED ON THE SUBSTRATE IN ACCORDANCE WITH THE DESIRED THICKNESS OF THE PROJECTION.
 2. The method of claim 1 wherein the sputtering of the substrate material from the surface of the substrate occurs selectively only over a predetermined portion of such surface to produce a cavity therein, and the sputtering of substrate and target material from the target onto the substrate is effective to fill such cavity and eventually to deposit material thereon to a level above that of the original substrate surface. 